Navigational systems for use in aircraft and other means of travel have included transmission and receiver devices for handling radio and radar information, which information is used to develop the basic data needed to nagigate. This data includes direction of travel, distance to or from fixed reference points, and the measurement of azimuth. The present invention relates to navigational receivers in general, and in particular, to an avionics system for receiving a plurality of signals such as OMNI mode VOR signals, ILS localizing signal (LOC) and ILS glide slope signals (GS) wherein a single frequency synthesizer may be utilized for controlling the operation of a voltage controlled oscillator for providing mixing frequencies to the radio frequency mixers of the receiver. Such a frequency synthesizer would be required to handle a large number of channel frequencies and quickly switch between channels.
Prior receivers, such as the circuit disclosed by Lundgreen, et al, U.S. Pat. No. 3,386,096 and Ferrara, U.S. Pat. No. 4,025,923 have employed separate frequency synthesizers or driving voltage controlled oscillators (VCO) for supplying frequencies to each RF mixer within the navigational receiver. These circuits have incorporated specific individual components for processing each specific type of signal. Prior receivers which were required to handle both VOR/LOC and GS information have included a duplication of similar circuits for processing each of the various types of signals.
In a navigational receiver for handling VOR/LOC information and GS information on a time-shared, multiplex operational basis wherein many of the components including the frequency synthesizer and voltage controlled oscillator have a dual dedication, i.e., are switched between one processing function and its attending frequencies and another processing function at other frequencies, it is desirable to have a chemical frequency selection scheme which can be easily reconfigured and can be switched quickly between one operating frequency and another.
An object of this invention is to provide a digital channel selection scheme for a general aviation navigational receiver.
A second object of this invention is to provide such a navigational receiver with a digital frequency synthesizer connected to the channel selection switches of this unit for controlling the operation of an associated voltage controlled oscillator for providing mixer frequencies.
Another object of this invention is to provide such a frequency synthesizer preprogrammed to each of the frequencies needed for each of the channels to be selected by the operation of the unit and capable of multiplex operation for alternately switching the associated VCO between LOC mixer frequencies and GS mixer frequencies.
A further object of this invention is to provide for such a channel selection and switching between frequencies in synchronism with a basic clock resident within the frequency synthesizer to minimize loss of information.